1. Field of the Invention
This invention relates to drop-on demand ink jet printing and more particularly to thermal ink jet printing wherein each ink droplet is ejected by conservation of momentum of a collapsing bubble of vaporized ink.
2. Description of the Prior Art
Ink jet printing systems are usually divided into two basic types, continuous stream and drop-on-demand. In continuous stream ink jet printing systems, ink is emitted in a continuous stream under pressure through one or more orifices or nozzles. The stream is perturbed, so that it is broken into droplets at determined, fixed distances from the nozzles. At the break-up point, the droplets are charged in accordance with varying magnitudes of voltages representative of digitized data signals. The charged droplets are propelled through a fixed electrostatic field which adjusts or deflects the trajectory of each droplet in order to direct it to a specific location on a recording medium, such as paper, or to a gutter for collection and recirculation. In drop-on-demand ink jet printing systems, a droplet is expelled from a nozzle directly to the recording medium along a substantially straight trajectory that is substantially perpendicular to the recording medium. The droplet expulsion is in response to digital information signals, and a droplet is not expelled unless it is to be placed on the recording medium. Drop-on-demand systems require no ink recovering gutter to collect and recirculate the ink and no charging or deflection electrodes to guide the droplets to their specific pixel locations on the recording medium. Thus, drop-on-demand systems are much simpler than the continuous stream type.
There are two basic propulsion techniques for the drop-on-demand ink jet printers. One uses a piezoelectric transducer to produce pressure pulses selectively to expel the droplets and the other technique uses thermal energy, usually the momentary heating of a resistor, to produce a vapor bubble in the ink, which during its growth expels a droplet. Either technique uses ink-filled channels which interconnect a nozzle and an ink-filled manifold. The pressure pulse may be generated anywhere in the channels or the manifold. However, the bubble generating resistor (hence the name bubble jet) must be located in each channel near the nozzle.
The thermal ink jet printers, sometimes referred to as bubble jet printers, are very powerful because they produce high velocity droplets and permit very close nozzles spacing for printing higher numbers of spots or pixels per inch on the recording medium. The higher the number of spots per inch, the better the printing resolution, thus yielding higher quality printing.
In thermal ink jet printers, printing signals representing binary digital information originate an electric current pulse of a predetermined time duration in a small resistor within each ink channel near the nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a vapor bubble. The ink at the orifice is forced out as a propelled droplet by the bubble. At the termination of the current pulse, the bubble collapses and the process is ready to start all over again as soon as hydrodynamic motion or turbulence of the ink stops. The turbulence in the channel generally subsides in fractions of milliseconds so that thermally expelled droplets may be generated in the kilohertz range.
Existing thermal ink jet printers usually have a printhead mounted on a carriage which traverses back and forth across the width of a stepwise movable recording medium. The printhead generally comprises a vertical array of nozzles which confronts the recording medium. Ink-filled channels connect to an ink supply reservoir, so that as the ink in the vicinity of the nozzles is used, it is replaced from the reservoir. Small resistors in the channels near the nozzles are individually addressable by current pulses representative of digitized information or video signals, so that each droplet expelled and propelled to the recording medium prints a picture element or pixel.
In U.S. Pat. No. 4,463,359, a thermal ink jet printer is disclosed having one or more ink-filled channels which are replenished by capillary action. A meniscus is formed at each nozzle to prevent ink from weeping therefrom. A resistor or heater is located in each channel at a predetermined distance from the nozzles. Current pulses representative of data signals are applied to the resistors to momentarily vaporize the ink in contact therewith and form a bubble for each current pulse. Ink droplets are expelled from each nozzle by the growth of the bubbles which causes a quantity of ink to bulge from the nozzle and break off into a droplet at the beginning of the bubble collapse. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium such as paper. The current pulses are shaped to prevent the meniscus at the nozzles from breaking up and receding too far into the channels, after each droplet is expelled. Various embodiments of linear arrays of thermal ink jet devices are shown, such as those having staggered linear arrays attached to the top and bottom of a heat sinking substrate and those having different colored inks for multicolored printing. In one embodiment, a resistor is located in the center of a relatively short channel having nozzles at both ends thereof. Another passageway is connected to the open-ended channel and is perpendicular thereto to form a T-shaped structure. Ink is replenished to the open-ended channel from the passageway by capillary action. Thus, when a bubble is formed in the open-ended channel, two different recording mediums may be printed simultaneously.
IBM Technical Disclosure Bulletin, Vol. 18, No. 4, September 1975 to Fisher et al discloses an ink-on-demand ink jet printer in which jet formation is triggered ultrasonically and the ink reservoir is an ultrasonic cavity which enhances the ultrasonic effects on the meniscus at the orifice. A high-voltage electrode having an orifice therein and an acceleration electrode sandwich the printing medium. A voltage on the order of 2-4 kilovolts is applied to the electrode with the orifice and a voltage of about 7 kilovolts is applied to the acceleration electrode. The voltage from the electrode with the orifice causes a meniscus to be formed at the ink reservoir orifice. When it is desired to expel a droplet, resonant frequency is applied to piezoelectric crystal forming part of the ink reservoir. The combined electrostatic and hydrostatic forces on the ink, when not at resonance, are not sufficient to cause leakage of the ink or formation of the droplet which travels through the electrode orifice and impinges on the printing medium.
U.S. Pat. No. 4,251,824 to Hara et al discloses a thermally activated liquid ink jet recording method which involves driving one or a group of heaters to produce vapor bubbles in ink-filled channels of a printhead which expel ink droplets. In FIGS. 7A and 7B, a single resistor is used for each channel to expel drops from nozzles thereof. A plurality of resistors in each channel are shown in FIG. 12 which are sequentially driven to expel droplets. In FIG. 2C, simultaneous driving of varying quantities of resistors in each channel expels droplets of varying diameters.
U.S. Pat. No. 4,410,899 to Haruta et al discloses a method of forming ink droplets by a heat generator which forms bubbles to expel the droplets, but the bubbles do not fill the channels, so that the ink is not totally separated from the nozzle even when the bubbles reach their maximum size.
U.S. Pat. No. 4,336,548 to Matsumoto discloses a thermal ink jet printing device in which the surface of the heat generating section is made to have a surface coarseness of from 0.05S to 2S measured in accordance with the Japanese Industrial Standard JIS-B-0601.
U.S. Pat. No. 4,339,762 to Shirato et al discloses a thermal ink jet recording method wherein the heat generating element has a construction which provides that the degree of heat generated is different from position to position along the heating surface of the heat generating element and the strengthen of the input signal to the heat generating element is controlled, thereby controlling the distribution of degree of heat supplied to the ink at the heating surface in order to achieve gradation of an image to be recorded.
An article entitled "Solid-State Scanning Ink Jet Recording" by Ichinose et al, IEEE, 1983 discloses an ink jet recording head with one slit-like opening through which a plurality of ink jet streams may be produced one stream for each of a linear array of individually addressable recording electrodes. The ink stream is emitted from the slit-like opening due to the electrically addressed recording electrode and a counter electrode located behind the recording medium. The ink stream strikes the recording medium and forms a printed dot or pixel thereon.
An article entitled "Drop Formation Characteristics of Electrostatic Ink jet Using Water-Based Ink" by Agui et al, IEEE Transactions on Electron Devices, Vol. ED-24, No. 3, March 1977, pages 262-266, discloses droplet formation characteristics of electrostatic ink jets using water-based ink. Ink droplets are generated by the balance between surface tension forces and the electrostatic attractive force at the tip of a nozzle produced by an acceleration electrode. Experimental results obtained by varying the applied voltage to the acceleration electrode and the pressure of the ink in a nozzle bearing capillary tube are reported.